1. Field of the Invention
The present invention relates to the structure of external electrodes of electronic components used for high-density mounting, and more particularly, the invention relates to an electronic component in which tin plating layers are formed as the outermost layers of the external electrodes.
2. Description of the Related Art
Electronic components, such as chip-type monolithic capacitors, are usually mounted on printed wiring boards, etc., by soldering the external electrodes formed on the exterior of chip base members. In such a case, in order to improve solderability relatively inexpensively, tin plating layers are often formed as the outermost layers of the external electrodes. This is because tin plating layers facilitate soldering as compared with nickel layers and copper layers, and also tin plating layers do not easily cause defects when the electronic components are mounted by reflow soldering, flow soldering, or the like.
However, a drawback of the tin plating layer is that, for example, if directly formed on a thick-film electrode provided on the surface of a ceramic base member, parts of the tin plating layer may not adhere to the thick-film electrode, or during soldering, the thick-film electrode may be melted and absorbed by the solder. In order to overcome such a drawback, a nickel underlying film or a nickel alloy underlying film is often formed under the tin plating layer.
Recently, it has become known that when an electronic component in which tin plating layers are formed as the outermost layers of the external electrodes is placed in an environment in which the temperature cyclically changes, whisker-like protrusions, which are called whiskers, are generated on the tin plating layers. The whiskers thus generated have a polycrystalline structure, have a length of approximately 100 xcexcm at most and are curved, in contrast to the known linear single-crystal whiskers which are generated from tin plating layers formed on Cu-based underlying metals. Hereinafter, in order to distinguish from the single crystal whiskers, whiskers generated in an environment in which the temperature cyclically changes are referred to as xe2x80x9ctemperature cycle whiskersxe2x80x9d.
If such temperature cycle whiskers are generated on external electrodes of electronic components mounted on a circuit board, a short circuit may occur between adjacent components or between the individual components and wiring patterns. In particular, the mounting density of electronic components is rapidly increasing and in some cases, the distance between adjacent electronic components is as short as approximately 200 xcexcm. The distances between the adjacent electronic components and between the individual components and wiring patterns will be further decreased as the mounting density of circuit components is further increased in the future, and therefore the possibility of a short circuit due to the temperature cycle whiskers will be increased even though the temperature cycle whiskers have a length of approximately 100 xcexcm.
It is not possible to ignore an electrical short circuit on a circuit board having such a high mounting density.
As a method for suppressing temperature cycle whiskers, tin alloy plating layers are used. In view of satisfactory wettability and facilitation of plating, alloys of tin and lead are often used. However, use of lead is undesirable due to the adverse effect on the environment. On the other hand, alloying of tin with metals other than lead is still in the research stage and stable production thereof is not yet expected.
As another method for suppressing temperature cycle whiskers, use of metals other than tin or tin alloys is under study. For example, a method of using a gold plating layer or a lead plating layer as the outermost layers of the external electrodes is under study. However, since these materials are noble metals, there will be a significant increase in costs.
Accordingly, it is an object of the present invention to provide an electronic component provided with external electrodes in which temperature cycle whiskers are not generated even in an environment in which high temperature and low temperature alternate.
As a result of thorough studies, the present inventors have discovered that temperature cycle whiskers, which are generated on tin plating layers in an environment in which the high-temperature state and the low-temperature state are alternately repeated, are formed because tin atoms in the tin plating layers concentrate on specific spots using the stress caused by the changes in temperature as a driving force. That is, the tin plating layer has a polycrystalline structure, and tin atoms are considered to migrate to the surface of the tin plating layer along tin crystal grain boundaries. The present inventors have found that it is possible to inhibit tin atoms from migrating by diffusing atoms of a metal other than tin into the tin crystal grain boundaries, thus suppressing the generation of whiskers.
In one aspect of the present invention, an electronic component includes external electrodes formed on a base member, each external electrode including a plurality of layers, the outermost layer of the plurality of layers being a tin plating layer, in which the tin plating layer has a polycrystalline structure, and atoms of a metal other than tin are diffused into the tin crystal grain boundaries.
In the tin plating layer having such a structure, whiskers are not generated even in an environment in which the temperature repeatedly changes even if stress occurs in the tin plating layer due to a change in temperature since the migration of tin atoms along the tin crystal grain boundaries is inhibited.
Preferably, the atoms of the metal other than tin are nickel atoms. Preferably, a nickel layer or a nickel alloy layer is formed under the tin plating layer. The nickel layer or the nickel alloy layer is formed as an outer layer of the external electrode, and the tin plating layer is further formed on the nickel layer or the nickel alloy layer, and then the external electrode is heat-treated under predetermined conditions. Thereby, a part of the nickel atoms in the nickel layer or the nickel alloy layer can be easily diffused into the tin crystal grain boundaries.
In another aspect of the present invention, an electronic component includes external electrodes formed on a base member, each external electrode including a plurality of layers, the plurality of layers including a thick-film electrode formed on the base member, a nickel layer or a nickel alloy layer formed on the thick-film electrode, and a tin plating layer formed on the nickel layer or the nickel alloy layer, in which the tin plating layer has a polycrystalline structure and nickel atoms are diffused into the tin crystal grain boundaries.
In another aspect of the present invention, a method for fabricating an electronic component includes the steps of forming a thick-film electrode on a base member; forming a nickel layer or a nickel alloy layer on the thick-film electrode; forming a tin plating layer on the nickel layer or the nickel alloy layer so as to produce an external electrode composed of a metallic film having a layered structure; and heat-treating the external electrode under predetermined conditions. Thereby, a part of the nickel atoms in the nickel layer or the nickel alloy layer can be easily diffused into the tin crystal grain boundaries.
In an electronic component fabricated in accordance with the present invention, temperature cycle whiskers, which have a length of approximately 100 xcexcm and which may be generated in an environment in which the temperature changes cyclically, are inhibited from occurring. Consequently, when a plurality of such electronic components are mounted on a circuit board, even if the distance between adjacent electronic components is decreased to 200 xcexcm or less, the possibility of a short circuit caused by the temperature cycle whiskers is decreased, thus enabling high-density mounting.